The invention relates to a process for determining the loading condition of a diode rectifier of a frequency converter connected to a mains supply.
In frequency converters, in particular in those where diode rectifiers are used to supply a DC link circuit, faults in the mains supply result in increased loading of the diode rectifier, which can be destroyed by an overload.
In particular, dips in the voltage from the mains supply with subsequent rapid restoration of the dipped mains voltage cause significant loading of the diodes of the diode rectifier of the frequency converter connected to the mains supply. The rapid restoration of the mains voltage means that the DC link circuit of the frequency converter connected to the mains supply is recharged by a very high charging current. Since a charging circuit for the DC link circuit provided in the frequency converter is not enabled for momentary mains voltage dips (<20 msec), the magnitude of the charging current after restoration of supply is set solely by the voltage difference between mains voltage and DC link voltage, and the rate at which the voltage is restored. The diodes of the rectifier must conduct this excessive charging current following restoration of supply. If there is a succession of such momentary mains voltage dips, then the diodes of the diode rectifier of the frequency converter connected to the mains supply are thermally damaged, causing the frequency converter to fail.
Such a problem is more common in mains supplies that are not very stable. Until now, this problem has been solved by oversizing the diode rectifiers of a frequency converter, i.e. the rectifier is designed to be large enough to withstand such a fault using a surge-current threshold value or maximum load integral. In addition, reactors having large inductance values are used on the line side. Line reactors are also required if mains pollution from the frequency converter is to be minimized This means that if these existing line reactors are also to be used to help solve the problem mentioned, their inductance values must be increased accordingly. If frequency converters are manufactured for specific countries, then no measures need to be taken if these frequency converters are only operated on very stable mains systems. Market globalization means that it is not economically viable if one and the same frequency converter is designed differently or is provided with options for different mains systems. It would be very useful for someone purchasing a frequency converter if the frequency converter could itself determine the loading condition of its line-side diode rectifier in order to be able to respond selectively, and in good time, to a loading condition that is increasing.